The vast continental shelf region extending along the north coast of Alaska and Canada in the Beaufort Sea is currently regarded as having some of the best potential for large new petroleum discoveries in North America. For this reason, the region is presently undergoing exploration activity. Offshore exploration activities are almost always more expensive than onshore exploration activities, but special problems faced in offshore arctic regions such as the Beaufort Sea result in some of the most expensive exploration activities ever undertaken, with the cost of drilling an exploratory well in some cases exceeding one hundred million dollars. For this reason, there has been great incentive to develop approaches for drilling wells in offshore arctic regions which are so cost effective as possible without subordinating safety and environmental concerns. Before describing some of the drilling approaches which have been used and proposed, a brief description of the unique problems encountered in offshore arctic drilling operations will be given.
The biggest problem facing drilling operations in offshore arctic regions such as the Beaufort Sea is the presence of sea ice. During the brief summer open water season, sea ice is generally not a problem, except when strong north winds blow large bodies of multiyear ice into the coastal regions from the permanent polar ice cap which lies far to the north. During the open water season, relatively conventional drilling vessels can be used to drill exploratory wells. However, the open water season is short, typically lasting for only two or three months. In bad ice years, the open water season can be even shorter. A further time constraint is imposed by governmental regulations which prohibit drilling during parts of the open water season.
During most of the year, the presence of sea ice makes the use of unprotected drilling vessels unsafe and impractical. The sea generally begins to freeze up in early October, leading to the gradual formation of a stationary landfast ice zone in nearshore waters. Beyond this landfast zone lies the transition zone. The transition zone extends from the landfast zone seaward to the permanent polar ice cap. Unlike the stationary ice in the landfast zone, the ice in the transition zone is highly mobile ice called pack ice. During the early part of the winter, the boundary of the landfast zone moves seaward, stabilizing near the sixty foot water depth contour in early Jan. For the most part, the stationary ice of the landfast zone and the pack ice of the transition zone consist of annual ice which starts out very thin and grows to a thickness of about six feet by mid winter. Embedded within the annual ice are much thicker ice features originating from the permanent polar ice cap or resulting from ice deformation caused by movements within the transition zone. Some of the thicker ice features are so large that they are able to survive the brief open water season, thereby becoming multiyear ice features.
Of the two annual ice zones, the landfast zone poses less problems to drilling operations because it is relatively stationary. Nevertheless, compressive forces which develop as the landfast ice thickens make the use of conventional drilling vessels unattractive. In addition, the boundary of the landfast zone can quickly recede toward shore under the influence of strong winds, especially in the early winter when the annual ice cover is not yet very thick. Thus, except in areas which are quite close to shore, the pack ice of the transition zone must usually be regarded as a threat.
What makes pack ice so threatening is its mobility. Whenever moving pack ice encounters a stationary object, compressive forces develop. In general, the thicker the ice, the greater the compressive forces. The greatest compressive forces develop when the large multiyear ice features embedded within the pack ice encounter stationary objects. Because of the compressive forces which can develop, unprotected drilling vessels have not been used for winter drilling operations. Even if the drilling vessel hull were made strong enough to resist the compressive forces, the mooring system might not be able to keep the vessel stationary, and the well being drilled could be lost as the vessel is pushed off location. Consequently, special approaches for winter drilling in the Beaufort Sea have been used and proposed.
The most common approach which has been used is to build a gravel island at the drilling location and to place a drilling rig on top of the gravel island. Gravel islands are capable of resisting the compressive ice forces due to their enormous mass and the inherent strength of gravel. As annual pack ice moves against a gravel island, it breaks and piles up, forming grounded ice rubble adjacent to the gravel island. This grounded ice rubble provides additional protection for the gravel island against large multiyear ice features embedded in the pack ice. Ideally, grounded ice rubble will form around the gravel island before any encroachment by multiyear ice. Although gravel islands have been used successfully to drill offshore arctic wells, they have a number of serious drawbacks. Gravel islands are extremely expensive to build and require a great deal of construction time.
Several variations of the gravel island approach have also been used. In one approach, a berm is constructed in essentially the same manner as a gravel island, except that construction of the berm ceases when its top is still well below the water's surface. The top is then leveled, and steel caissons are placed on top of the berm to define an enclosed area. The steel caissons extend above the water's surface and together form a ring. Gravel or sand is dumped into the caisson ring to build up a caisson retained island. This approach has the advantage of requiring less fill material and of avoiding the severe wave erosion problems faced by gravel islands. A similar approach is to build an underwater berm with a level top and then to ballast a steel drilling structure onto the top of the berm. The strong steel sides of the structure resist compressive forces imposed by moving pack ice and the ballast prevents the pack ice from pushing the structure off the berm. In this approach, even less fill material is required, but the cost savings are at least partially offset by the greater cost of the drilling structure relative to a caisson ring. Despite their advantages over gravel islands, both of these berm approaches are still very expensive and require a great deal of construction and preparation time before drilling can begin.
Another approach to offshore arctic drilling is to use a massive bottom-founded drilling structure. The strong sides of the structure resist crushing forces, and the mass and foundation of the structure keep it from being pushed off location. One advantage of this approach over those previously described is that time delays are minimal since no gravel island or berm has to be constructed. Another advantage is that unlike a gravel island or berm, the structure can be moved to drill at more than one location. However, like the drilling approaches which use gravel islands and berms, the use of a bottom-founded drilling structure is very expensive.
One approach which has been proposed for offshore arctic drilling is to use a relatively inexpensive drilling vessel protected by a spray ice barrier. A horseshoe-shaped spray ice barrier would be constructed using high output spray monitors which shoot streams of seawater through the air at times when the air is below the freezing temperature of the seawater, thus forming spray ice. Thousands or millions of tons of spray ice would be deposited in this manner until a grounded spray ice barrier has been constructed. The drilling vessel would then be moved into the protected enclosure formed by the horseshoe-shaped barrier. The enormous mass of the grounded spray ice barrier would enable it to resist the moving pack ice and to thereby protect the drilling vessel.
The spray ice barrier approach holds much promise because it is far less expensive than the winter drilling approaches described above. Thanks to the low cost and the rapid vertical buildup rates achievable with the spray ice barrier construction method, this approach holds more promise than gravel islands and berms for offshore arctic drilling in deeper waters. The spray ice barrier approach is also far less expensive than the bottom-founded drilling structure approach described above. To date, spray ice barriers have been used to provide additional protection for bottom-founded and berm-founded drilling structures and caissons. This experience may soon lead to the use of drilling vessels protected by spray ice barriers for offshore arctic drilling. Nevertheless, the arctic drilling vessels which would be used in the spray ice barrier approach are still quite expensive, and their availability may be limited for some time to come.
The high cost of arctic gravel islands, berms, bottom-founded drilling structures and drilling vessels has led to the development of approaches which rely on drilling platforms made of ice. One approach which has been used is to construct an ice island made of solid ice which is formed by a flood-and-freeze process. Thin layers of water are deposited on top of annual ice which forms at the drilling location. Each layer of water is allowed to freeze before more water is added. The flood-and-freeze process is repeated until an ice island of desired thickness has been constructed. Because the ice island must not be allowed to move during drilling, construction usually continues until the ice island becomes grounded on the sea floor. However, due to the slow vertical buildup rates achievable with the flood-and-freeze process, constructing a grounded ice island by this process is practical only in shallow water. Ungrounded ice islands have been used, but they are practical only in some relatively unusual areas of deep water landfast ice, such as between the closely-spaced Arctic Islands of Canada. Thus, the use of ice islands constructed by the flood-and-freeze process is limited to a very small percentage of the offshore arctic regions currently being explored.
The limitations imposed by the slow vertical buildup rates achievable with the flood-and-freeze process have led to proposals to build drilling platforms out of spray ice. A grounded spray ice drilling platform would be constructed in much the same manner as a spray ice barrier, taking advantage of the rapid vertical buildup rates achievable with the spray ice process. Unlike spray ice barriers however, a spray ice drilling platform must have a good working surface. To date, no operational grounded spray ice drilling platforms have been constructed, but grounded spray ice relief well platforms have been constructed next to caisson retained islands for use in the event of a blowout of the well being drilled from the island. Because no blowouts have occurred, these spray ice relief well platforms have not been used for drilling. In most cases, spray ice relief well platforms have been built by first leveling grounded ice rubble which forms next to caisson retained islands. Spray ice is then deposited intermittently, allowing earth moving equipment to level and compact the spray ice in layers. The grounded ice rubble provides a firm foundation for the spray ice, so that spray ice only has to be added on top of the grounded ice rubble to form the platform. This is highly advantageous because spray ice has a tendency to settle and crack. The greater the volume of spray ice that has to be used to construct the platform, the greater the tendency for settlement and cracking. The presence of a grounded ice rubble foundation minimizes the volume of spray ice that has to be used to construct the platform, thereby minimizing settlement and cracking.
Another advantage of building spray ice drilling platforms on top of grounded ice rubble results from the greater density of the ice rubble relative to spray ice. This gives the platform more mass for the same height and hence increases sliding resistance. A spray ice platform constructed on floating ice could of course be built to a higher freeboard to achieve the same sliding resistance. Unfortunately however, this higher freeboard would aggravate the settlement and cracking problem due to the greater volume of spray ice required. In addition, the higher freeboard would make access to the spray ice drilling platform difficult.
A further advantage of building spray ice drilling platforms on top of grounded ice rubble results from the fact that ice rubble has greater compressive strength than spray ice. Thus, a spray ice drilling platform built on top of grounded ice rubble is less likely to be deformed by pack ice movements than a spray ice drilling platform lacking such a foundation. For obvious reasons, any deformation of a drilling platform should be avoided.
All of the advantages of building spray ice drilling platforms on top of grounded ice rubble foundations can only be realized if grounded ice rubble forms at the desired drilling location. In the case of spray ice relief well platforms used in conjunction with stationary drilling structures such as caisson retained islands, odds are good that grounded ice rubble will form next to the stationary structure. However, the odds that grounded ice rubble will form at a desired drilling location in the absence of a stationary structure are low, especially in deeper waters. Drilling plans of course cannot be based on such low odds. Therefore, if spray ice drilling platforms are ever to realize their potential, an approach will have to be developed for overcoming the settlement, cracking, sliding resistance and compressive strength problems facing spray ice structures which are constructed on floating ice. The present invention is aimed at providing such an approach.